Organic light emitting devices (OLEDs) represent a promising technology for display applications. A typical organic light emitting device includes a first electrode; a luminescent region comprising one or more electroluminescent organic material(s); and a second electrode; wherein one of the first electrode and the second electrode functions as a hole-injecting anode, and the other electrode functions as an electron-injecting cathode; and wherein one of the first electrode and the second electrode is a front electrode, and the other electrode is a back electrode. The front electrode is transparent (or at least partially transparent) while the back electrode is usually highly reflective to light. When a voltage is applied across the first and second electrodes, light is emitted from the luminescent region and through the transparent front electrode. When viewed under high ambient illumination, the reflective back electrode reflects a substantial amount of the ambient illumination to the observer, which results in higher ratios of reflected illumination as compared to the device's own emission, resulting in “washout” of the displayed image.
In order to improve the contrast of electroluminescent displays in general, light absorbing layers as described, for example, in U.S. Pat. No. 4,287,449, or optical interference members as described, for example, in U.S. Pat. No. 5,049,780, have been used to reduce the ambient illumination reflection.
Another problem of known organic light emitting devices originates from the use of metals with low work functions, and hence high reactivity, in the cathodes. Due to their high reactivity, such cathode materials are unstable in ambient conditions and react with atmospheric O2 and water to form non-emissive dark spots. See, for example, Burrows et al., “Reliability and Degradation of Organic Light Emitting Devices,” Appl. Phys. Lett. Vol. 65, pp. 2922-2924 (1994). To reduce such ambient effects, organic light emitting devices are typically hermetically sealed, immediately after fabrication, under stringent conditions, such as, for example, less than 10 ppm moisture atmospheres.
Thus, there is a need which the present invention addresses for new display devices that avoid or minimize a number of the above mentioned problems. In particular, as described herein, the present display devices provide in embodiments a reduced light reflection.
Other documents that may be relevant to the present invention include the following:    Liang-Sun Hung et al., “Reduction of Ambient Light Reflection in Organic Light-Emitting Diodes,” Advanced Materials Vol. 13, pp.1787-1790 (2001);    Liang-Sun Hung et al., U.S. application Ser. No. 09/577,092 (filed May 24, 2000);    EP 1 160 890 A2 (claims priority based on above U.S. application Ser. No. 09/577,092;    Japanese laid open patent document No. 8-222374 (laid open date Aug. 30, 1996);    O. Renault et al., “A low reflectivity multilayer cathode for organic light-emitting diodes,” Thin Solid Films, Vol. 379, pp.195-198 (2000);    WO 01/08240 A1;    WO 01/06816 A1;    David Johnson et al., Technical Paper 33.3, “Contrast Enhancement of OLED Displays,” http://www.luxell.com/pdfs/OLED_tech_ppr.pdf, pp. 1-3 (April 2001);    Junji Kido et al., “Bright organic electroluminescent devices having a metal-doped electron-injecting layer,” Applied Physics Letters Vol. 73, pp.2866-2868 (1998);    Jae-Gyoung Lee et al., “Mixing effect of chelate complex and metal in organic light-emitting diodes,” Applied Physics Letters Vol. 72, pp.1757-1759 (1998);    Jingsong Huang et al., “Low-voltage organic electroluminescent devices using pin structures,” Applied Physics Letters Vol. 80, pp.139-141 (2002);    L. S. Hung et al., “Sputter deposition of cathodes in organic light emitting diodes,” Applied Physics Letters, Vol. 86, pp. 4607-4612 (1999);    EP 0 977 287 A2;    EP 0 977 288 A2;    Hany Aziz et al., U.S. application Ser. No. 09/935,031, filed Aug. 22, 2001.
Other documents that may be relevant to the present application were submitted in parent U.S. application Ser. No. 09/800,716 (filed Mar. 8, 2001), such other documents being:    U.S. Pat. No. 4,885,211;    U.S. Pat. No. 5,247,190;    U.S. Pat. No. 4,539,507;    U.S. Pat. No. 5,151,629;    U.S. Pat. No. 5,150,006;    U.S. Pat. No. 5,141,671;    U.S. Pat. No. 5,846,666;    U.S. Pat. No. 5,516,577;    U.S. Pat. No. 6,057,048    U.S. Pat. No. 5,227,252;    U.S. Pat. No. 5,276,381;    U.S. Pat. No. 5,593,788;    U.S. Pat. No. 3,172,862;    U.S. Pat. No. 4,356,429;    U.S. Pat. No. 5,601,903;    U.S. Pat. No. 5,935,720;    U.S. Pat. No. 5,728,801;    U.S. Pat. No. 5,942,340;    U.S. Pat. No. 5,952,115;    U.S. Pat. No. 4,720,432;    U.S. Pat. No. 4,769,292;    U.S. Pat. No. 6,130,001;    Bernius et al., “developmental progress of electroluminescent polymeric materials and devices,” SPIE Conference on Organic Light Emitting Materials and Devices III, Denver, Colo., July 1999, SPIE, Vol. 3797, pp. 129-137;    Baldo et al., “highly efficient organic phosphorescent emission from organic electroluminescent devices,” Nature Vol. 395, pp. 151-154 (1998);    Kido et al., “white light emitting organic electroluminescent device using lanthanide complexes,” Jpn. J. Appl. Phys. Vol. 35, pp. L394-L396 (1996);